While the use of extrusion for producing plastic panels of various cross-sections is known, such panels are at best translucent, but not transparent. For optical-quality transparency, extruded panels have to undergo a calendering process, when they are passed between several rolls having a hard, highly-polished surface. Optical-quality surfaces are therefore contingent on "rollability", and the demand for rollability imposes certain constraints and limitations on surface configuration (quite apart from the general rollability condition of the absence of undercuts which holds for all rolling operation, regardless of surface quality), namely, the specific condition of an absence of friction between rolling and rolled surfaces. Technically, this translates into the condition that the surface speed of all points of active roll surfaces be identical, which obviously means that the radii of these points be identical, too, as surface speed at a given rotational speed is a function of radius.
Another constraint of basic rolling techniques is the impossibility of producing flanged panels in which the flange surfaces have features such as grooves, serrations, teeth, etc.